In an image forming apparatus that adopts a developing system of transferring electrified toner onto a transferred body, a developer in which toner and carrier are mixed is stirred so that the toner electrified by friction.
In addition, only electrified toner of the developer is carried on a surface of a photoconductive drum and is transferred onto the transferred body, and the carrier remains in the developing device.
Accordingly, the toner concentration of the developer stored in the developing device is reduced by repetition of processing of developing an image.
For this reason, a developer is supplied to the developing device from an external toner tank when the concentration of toner becomes smaller than a threshold value.
This supply is performed including not only toner but also carrier because a coat agent of a carrier is pealed off or dropped or an additive of toner adheres to the carrier surface due to stirring in developing device.
In addition, an old surplus developer (developer with reduced toner concentration) generated by supply of the new developer is discharged from the developing device and is then collected (for example, JP-A-9-185177).
Meanwhile, in a transport path along which the surplus developer is discharged from an outlet of the developing device and reaches a collection box, there exists a place where the developer flows by gravity.
However, the flowability of the developer in the transport path may be stopped by a decrease in fluidity of the developer caused by abrasion and deterioration of the carrier.
Thus, when the smooth flow of the surplus developer to the collection box is obstructed, the outlet is clogged and the amount of developer stored in the developing device increases up to a value larger than a defined value. As a result, a problem that the quality of a formed image is lowered occurs.